Conventionally, an example of an exhaust gas recirculation device for an internal combustion engine as described in Patent Document 1 has been known. An exhaust gas recirculation device as disclosed in Patent Document 1 changes the opening degree of an exhaust gas recirculation valve provided in an exhaust gas recirculation passage such that the recirculation amount of exhaust gas is appropriate for the operating state of the engine.
An abnormality can occur in the above described exhaust gas recirculation device due to a failure in the operation of the exhaust gas recirculation valve or clogging of the exhaust gas recirculation passage. When there is an abnormality in the exhaust gas recirculation device, the device cannot adjust the recirculation amount of exhaust gas to an amount appropriate for the engine operating state.
To diagnose whether there is an abnormality in an exhaust gas recirculation device, the following abnormality diagnosis may be executed. That is, when a condition for executing the abnormality diagnosis is met, the opening degree of the exhaust gas recirculation valve is forcibly changed to the design minimum opening degree, and changes in a state quantity related to intake due to changes in the opening degree of the exhaust gas recirculation valve, for example, the amount of change in the intake pressure is monitored. When the amount of change in the intake state quantity is less than a predetermined determination value, it is determined that there is an abnormality in the exhaust gas recirculation device. That is, this method for diagnosing abnormalities takes advantage of the fact that the intake state quantity changes under the influence of changes in the exhaust gas recirculation amount. When there is an abnormality in the exhaust gas recirculation device, the amount of change in the actual exhaust gas recirculation amount does not correspond to the opening degree of the exhaust gas recirculation valve, and therefore, the amount of change in the intake state quantity does not correspond to changes in the opening degree of the exhaust gas recirculation valve. Therefore, if the amount of change in the intake state quantity is less than the predetermined determination value when the opening degree of the exhaust gas recirculation valve is changed to the minimum opening degree, it is determined that there is an abnormality in the exhaust gas recirculation device.
Another type of known exhaust gas recirculation device executes adjustment (opening degree adjustment) for reducing the opening degree of an exhaust gas recirculation valve when the density of intake air is low. For example, the device reduces the opening degree of the valve when the vehicle is at a high altitude compared to a case where the vehicle is at a low altitude. The execution of the abnormality diagnosis in such an exhaust gas recirculation device might bring about the following drawbacks.
That is, when the adjustment of the opening degree of the exhaust gas recirculation valve is being executed, the opening degree of the exhaust gas recirculation valve at the start of the abnormality diagnosis is maintained at a relatively small value. Therefore, even if the opening degree of the exhaust gas recirculation valve is forcibly changed from the opening degree at the start of the abnormality diagnosis to the minimum opening degree, the amount of accompanying change in the intake state quantity is small. Therefore, even if there is no abnormality in the exhaust gas recirculation device, the amount of change in the intake state quantity falls below the determination value of the abnormality diagnosis. This might lead to a false determination of an abnormality in the exhaust gas recirculation device.
So far, disadvantages of the abnormality diagnoses executed during the opening degree adjustment of the exhaust gas recirculation valve have been discussed. However, such disadvantages are present not only in the above described condition, but also in a case where the opening degree of the exhaust gas recirculation valve is set to a small value while the density of the intake air is high, for example, when the vehicle is at a low altitude.    Patent Document 1: Japanese Laid-Open Patent Publication No. 6-288303